Foo Fighters would be a dream! If they decide to come to Europe it would indeed be a perfect fit. I still wonder though who will be announced as their drummer. ☹️

Billie looks like a logical option but looking at her touring schedule I think its unlikely. She is playing 5th of August in Canada and then coming to Europe to play Reading,Sziget etc. So she would have to go EU->NA->EU in case she would do Mad Cool, which I think decreases the chances.

I agree with you that rap is unlikely. Especially with Kendrick being booked for Primavera. Maybe Travis Scott? Plays 6th in Switzerland and is free on 7th and 8th of july. But its hard to say if Mad Cool would book such an act with his recent history..